A cellular phone system has been put to practical use as a wireless system that provides a voice communication service, and, recently, various data services, such as e-mails and Internet access, have become available in addition to the voice communication service. The spread of a broadband wireless communication makes it possible to transmit and receive higher-definition video information, and new services are expected to be created.
Although the capacity in a wireless communication system has become larger, and the throughput in wireless communication has become higher through the development of wireless communication technology, there remains a big problem. The problem is that a service area becomes much smaller in a broadband wireless communication system than a conventional wireless system in which voice communication service is provided.
This problem mainly results from the fact that, when voice is transferred by using the broadband wireless system, voice having a frequency bandwidth of at most several kilohertz is transferred by using a frequency bandwidth of several tens of megahertz, and therefore excessive noise occurs in a receiver. This noise can be reduced by using baseband processing by use of redundancy. However, if such baseband processing is used, a very low-transmission rate wireless communication will be performed while using a frequency bandwidth of several tens of megahertz, and the wireless communication system will become extremely low in transmission efficiency.
There is also OFDMA (Orthogonal Frequency Division Multiple Access) technology that can provide a combination of a broadband service and a narrowband service, such as a voice communication service. However, even using the OFDMA technology, the communication area of the broadband service is provided in very narrow area.
Although both a voice communication service and a data service have been already provided by using a wireless LAN conforming to IEEE 802.11 that is the only broadband wireless communication system being in practical use at the present time, these services using the wireless LAN conforming to IEEE 802.11 have not been satisfactorily spread because there is a bottleneck in the fact that the service area of the voice communication service is narrow.
An area expanding technology developed by, for example, Xirrus, Inc. of the United States has already existed as the technology of expanding the service area of the wireless LAN conforming to IEEE 802.11, and a wireless base station provided with this technology has already been commercialized.
A block diagram of a wireless base station based on the technology of Xirrus Inc. is shown in FIG. 35. This station has sixteen access points (AP), each of which has a directional antenna. Directivity of each directional antenna of the AP differs from each other. Different frequency channels from each other are assigned to the access points, respectively. A controller controls the access points and the sixteen directional antennas. According to the structure of FIG. 35, it is reported that an antenna gain of about 12 dB can be obtained, and the radius of the service area can be extended about twice as long as a structure in which an omnidirectional antenna is used.
Area expanding technology developed by Ruckus Wireless Inc. of the United States exists as another example of the technology of expanding the service area of the wireless LAN, and, likewise, a wireless base station provided with this technology has already been commercialized. This technology is disclosed in Patent Literature 1.
A block diagram of a wireless base station based on the technology of Ruckus Wireless Inc. is shown in FIG. 36. The technology of Ruckus Wireless Inc. is the same as that of Xirrus, Inc. in the fact that the wireless base station uses sixteen directional antennas and that the service area is widened by the antenna gain. However, the technology of Ruckus Wireless Inc. differs from that of Xirrus, Inc. in the fact that an access point for each antenna is not provided and that the antennas are used while appropriately changing a combination of these antennas by use of a radio frequency switch. What is required of this structure is to provide one access point for each wireless base station, and therefore costs and power consumption can be reduced.
If the service-area expanding technology of the wireless LAN mentioned above is applied to the wireless base station that is to say the access point of the broadband wireless communication system, the radius of the service area can be extended several times. Nevertheless, its service area will be smaller than that of the current narrow band wireless communication system which is used mainly for a voice communication service.
As a concrete example, a description will be given of the service area of a wireless LAN conforming to IEEE 802.11a (hereinafter, referred to as an “IEEE 802.11a system”) and the service area of the current wireless communication system (PHS system) which is used mainly for a voice communication service.
FIG. 37 shows a relationship between a wireless-base-station-to-wireless-terminal distance (i.e., distance between a wireless base station and a wireless terminal) and received-signal power in the IEEE 802.11a system and in the PHS system, and shows reception sensitivity in each system. Herein, an indoor propagation model of ITU-R P.1238-2 was used as a radio propagation model, and general values in product specifications were used as specifications of each system. As shown in FIG. 37, the received signal power is reduced in proportion to an increase in the wireless-base-station-to-wireless-terminal distance. The radius of the service area of each system is obtained as a wireless-base-station-to-wireless-terminal distance when the received signal power coincides with the reception sensitivity of that system.
The received signal power given by the radio propagation model is a mean value of the received signal power. Actually, a variation by Rayleigh fading is superimposed on that model. Therefore, if the radius of the service area is determined by use of the mean value of the received signal power, the received signal power will fall below the reception sensitivity with an about ½ probability near the boundary of the service area. Preferably, in order to prevent this, under the condition that a variation by Rayleigh fading is superimposed, an electric power value (hereinafter, referred to as a “1% value”) in which the cumulative probability of received-signal power becomes 1% is calculated, and a wireless-base-station-to-wireless-terminal distance obtained when the resulting value coincides with the reception sensitivity of the system is set as the radius of the service area.
In FIG. 37, the mean value of received signal power is shown by the broken line, and the 1% value thereof is shown by the solid line. The wireless-base-station-to-wireless-terminal distance at a point at which the solid line (1% value) intersects with the straight line showing the reception sensitivity of each system is the radius of the service area. From FIG. 37, the radius of the service area of the IEEE 802.11a system and that of the PHS system are 18 meters and 114 meters, respectively. In each system, the probability with which the received signal power falls below the reception sensitivity inside the service area is less than 1%, and an excellent speech quality can be obtained there.
As is apparent from the foregoing description, the radius of the service area of the IEEE 802.11a system is about ⅙ of that of the PHS system, and, in terms of area ratio, the IEEE 802.11a system is about 1/40 of the PHS system. At the boundary of the service area of the PHS system (i.e., point at which the wireless-base-station-to-wireless-terminal distance is 114 meters), the 1% value of the IEEE 802.11a system is about −111 dBm, which is lower by 24 dB than −87 dBm that is the reception sensitivity of the IEEE 802.11a system. In other words, in order to achieve a service area having a size equivalent to that of the current PHS system, the IEEE 802.11a system is required to make a 24 dB dynamic-range improvement.
A dynamic-range improvement obtained when the service-area expanding technology mentioned above is applied to the wireless base station is at most 12 dB (corresponding to power gain 16) even when sixteen directional antennas, for example, are used. From FIG. 37, it is understood that a dynamic-range improvement of 12 dB makes it possible to extend the service-area radius to 42 meters. Thus, the service-area radius can be extended to more than twice as long as the service-area radius of the current IEEE 802.11a system by applying the service-area expanding technology mentioned above. Nevertheless, the service-area radius of the IEEE 802.11a system is about ⅓ of that of the PHS system, and the area of the IEEE 802.11a system is about ⅛ of that of the PHS system, and hence the IEEE 802.11a system remains small in the service area.
In order to obtain a service area equivalent to that of the PHS system, the wireless base station is required to be provided with 256 or more directional antennas (correspond to a gain of 24 dB). However, this is not realistic. A great dynamic-range improvement of about 24 dB can also be achieved by providing each of the wireless base station and the wireless terminal with at least 16 directional antennas (corresponding to a gain of 12 dB). However, a dimensionally wide space is required in order to form a directional antenna having a high gain of about 12 dB, and hence the volume of the wireless terminal must also be increased in order to mount 16 directional antennas each of which has such a high gain thereon.
Additionally, if radio waves come from various directions under a multipath environment, and all of these radio waves are intended to be received, the antenna gain will disadvantageously become small. In other words, in the technology developed by Ruckus Wireless Inc. in which directional antennas are used in a combined manner so as to achieve optimal directivity, the antenna gain becomes smaller in proportion to the approach of the probability distribution of the arrival angle of radio waves to uniform distribution, and, disadvantageously, the dynamic-range improvement becomes lower than 24 dB. On the other hand, although such a problem does not occur in the technology developed by Xirrus, Inc., 16 wireless transceivers correlated with 16 access points are required to be mounted on wireless terminals, and, disadvantageously, costs and power consumption are raised.
As described above, in the broadband wireless communication system, a very great dynamic-range improvement of about 24 dB is required in order to provide a voice communication service in the same service area as the conventional narrow band wireless communication system in addition to a wireless communication having a very high throughput of several tens of megabits per second. However, a technology that can achieve this improvement does not exist at the present time.
In recent years, in the field of the broadband wireless communication, a system that has introduced the MIMO technology has been generalized with the aim of increasing a transmission capacity or enhancing reliability. Although a system conforming to IEEE 802.11a or IEEE 802.11g based on the OFDM (Orthogonal Frequency Division Multiplexing) technology is dominant in, for example, a wireless LAN conforming to the IEEE 802.11 standard that has continued to spread in homes and offices under the present situation, IEEE 802.11n that is a new standard having introduced the MIMO technology has also been formulated. Recently, products equipped with the MIMO technology conforming to a draft of IEEE 802.11n have also begun to be released. The draft of IEEE 802.11n is shown in Non-Patent Literature 1.
Additionally, in the near future, the MIMO technology is also intended to be introduced in standards, such as mobile WiMAX or next-generation PHS, that will be introduced in a 2.5-GHz-band broadband mobile access system that is scheduled to be launched. This is described in Non-Patent Literature 2.
Still additionally, in the cellular phone, the MIMO technology is expected to be introduced into broadband-enabled “Super 3G” (name of the next-generation cellular system of NTT DOCOMO) and broadband-enabled “Ultra 3G” (name of the next-generation cellular system of KDDI) and into their later versions. Thus, it is predicted that the MIMO technology will be rapidly expanded with the spread of the broadband wireless communication.
As described above, in the broadband wireless communication, the introduction of the MIMO technology will probably be generalized in the future. It is known that a very large antenna gain and a very large diversity gain can be obtained if the MIMO technology is applied with a large number of antennas.
In the wireless communication system to which the MIMO technology is applied, signals transmitted from plural antennas of an opposite wireless communication device disposed on an opposite side of a communication link are received by plural antennas, thereafter each of the received signals is multiplied by diversity combining information (complex weight), and the signals are added together, thus generating a diversity-combining received signal. Optimal diversity combining information with respect to the signal of each antenna is calculated based on a channel matrix that is estimated from, for example, an already-known training sequence of received signals. The diversity-combining received signal is obtained by combining together signals the number of which is (the number of antennas of the opposite communication device×the number of antennas of the own wireless communication device). If the number of antennas of the opposite wireless communication device is N, and the number of antennas of the own wireless communication device is N, the number of signals combined together in the diversity combination is proportional to N2, and therefore a large MIMO gain can be obtained by increasing the number N.
FIG. 38 is a view showing a relationship between the number N of antennas and a MIMO gain in a case in which a wireless channel between a wireless base station and a wireless terminal is for over-the-horizon propagation (also termed “non-line-of-sight propagation”) and in which received signal power conforms to Rayleigh distribution. In FIG. 38, the abscissa axis exhibits received signal power standardized by the mean value of the received signal power when N=1, and the ordinate axis exhibits the cumulative probability distribution of received signals. The sum total of transmitted signal power from antennas is assumed as constant without depending on the number N of the antennas. As shown in FIG. 38, the graph of the cumulative probability distribution not only shifts rightwardly, but also inclines more steeply in proportion to an increase in the number N of the antennas. The rightward shift of the graph results mainly from an antenna gain in transmission and reception, whereas the change in inclination of the graph results mainly from a diversity gain. In comparison with a case in which N=1 at the 1% value, the MIMO gain is increased by 18 dB when N=2, and the MIMO gain is increased by 27 dB when N=4, and the MIMO gain is increased by 32 dB when N=8. From this fact, it is understood that, in a Rayleigh environment, a great dynamic-range improvement that exceeds 24 dB can be obtained by setting both the number of the antennas of the wireless base station and the number of the antennas of the wireless terminal at four or more.
As described above, in the broadband wireless communication system, a huge dynamic-range improvement that exceeds 24 dB is required in order to provide a video communication service in the same service area as the conventional narrow band wireless communication system in addition to a wireless communication having a high throughput of several tens of megabits per second. In order to achieve this improvement, the MIMO technology with four or more antennas is required to be applied onto both the wireless base station and the wireless terminal.
On the other hand, in the MIMO technology, it is necessary to optimally set diversity combining information with respect to the signal of each antenna and an eigenbeam-transmission line. This optimal diversity combining information has been obtained according to a conventional process in which the correlation matrix of a channel matrix is first calculated, and then an eigenvector corresponding to the maximum eigenvalue of the correlation matrix is calculated, and is set as diversity combining information with respect to the signal of each antenna.
Herein, the calculation amount in calculation of the correlation matrix increases in proportion to the square of the number of antennas, and the calculation amount in calculation of the eigenvector increases in proportion to the cube of the number of antennas. The calculation amount to calculate optimal diversity combining information sharply increases correspondingly to an increase in the number of antennas in this way, and therefore the increase in the number of antennas is limited.
Additionally, if the maximum frequency of multipath fading is assumed as 50 Hz (corresponding to ten kilometers per hour), transmission must be performed within several milliseconds from transmission-line matrix estimation in order to restrict a transmission-line estimation error to a small one, and a calculation to the eigenvector must be performed during the several milliseconds. If the number of antennas is five or more, the eigenvector cannot be analytically calculated and found, and a solution through an iterative process is required, and hence calculation time therefor becomes larger and it may come degradation of eigenvector.
Additionally, although a very large gain can be obtained by performing eigenbeam transfer by use of MIMO, information about a transmission line matrix must be shared between transmission and reception in order to perform eigenbeam transfer, and therefore information thereabout is required to be transferred. The amount of information about a transmission line matrix increases in proportion to an increase in the number of antennas. In a high-speed multipath fading environment, information thereabout is required to be frequently transferred, and the efficiency of the whole of the system is greatly deteriorated.
From the foregoing viewpoint, the upper limit of the number of antennas is four or so under the present situation. Actually, the maximum number of antennas in MIMO transfer is four in IEEE 802.11n or IEEE 802.16 that is a standard of wireless communication into which the most advanced MIMO technology of today has been introduced.
Additionally, diversity combining is performed by baseband processing in the conventional MIMO technology, and therefore an input signal of a transmitting/receiving circuit becomes a received signal prior to diversity combining In other words, in an input signal of a transmitting/receiving circuit, neither a diversity gain nor a MIMO gain can be obtained. The received signal is required to be detected in the transmitting/receiving circuit without transmission-beamforming at the initial acquisition stage before forming an eigenbeam. Therefore, in the conventional MIMO technology, the communication area has been limited to an area in which an eigenbeam can be formed by initial acquisition.